The User is a entity of the Warp a demon of Tzeetch it seduces and shape shifts into what ever to complete a task or just for its own desires.
The User was first known to seduce a Death Korps of Krieg guardsmen in charge of a heavy weapon shortly before a attack on a Tau held position. The User made the guardsmen mow down his comrades in front and behind. The User then was seen shape shifting to a Commissar that it killed hours before and killing that guardsmen that it just seduced to kill some comrades of him. The guard that saw it shape shift fired at the morphed daemon, the guard survived the enconter and the Inquisition 'interrogated' (tested on his eyes and memory) to understand it.
The user then cropped up on the other side of Imperial space in the Merilian sector specifically on Xerxes home world of the Brothers of Solitude. Though this was before they arrived. The User is believed to of corrupted the planet's governor and PDF leaders, the User then is thought to open the portal's that allowed the World Bearers to set foot on the World. As the Brothers of Solitude landed. The User disappeared once again.
The User has been known to hinder any force in the galaxy but likes to annoy any Imperial forces. It has stopped a whole regiment of guard by acting as a inquisitor of the Ordo Hereticus and killing the crew of a small ship, and ordering that a Orbital bombardment on the guard regiment accusing them of heresy.
The most obvious is its shape shifting and it uses it to great effect but it has been known to not shape shift in to space marines, Ork nobs and larger, Tyranids, Tau ethereal or any well spoken Tau army officers or officials, Imperial Guard officers above colonel, Necrons, Eldar and Dark Eldar which are high enough to notice, etc.
When in a certain form it can make objects work like star ships and weapons etc.
It can get in to people's dreams and has been known to turn them insane and ordering that person to to something more or less indirectly.
It also can help beings it trusts, it has been known to help Dark Eldar and greedy imperial governor's.
The User does not have a appearance per say it morphs in to them usual dead people so no ones any the wiser but it has no known original appearance.
It has appeared as many people of all race here lists just a few.
Imperial Guard Gunner heavy weapons.
Tau fire warrior
Tank commander baneblade (what you expect love the bane blade)
(Feel free to add your own)
(Feel free to add your own)
Inquisition, Or do Hereticus and Malleus
"Hmm baby it's a hard job serving the Emperor do you want me to release some of that pressure?....Done now in return do one thing for me....What is it? Well I want you to blow up your ammo stores....destroy the ship? well I'll save you"
-The user in a seductive female form seducing a Imperial Navy personal-
"Hahaha I'm in your head I'm in your head...Get out? Oh I will if you order a artillery strike on your armoured company in the woods setting an ambush...You will, good General I'll go as soon as you do it"
-The User in the mind of a Imperial Guard General the General committed suicide before giving the order-